In present day data processing systems, it is desirable to provide a large amount of memory which can be accessed in a minimal amount of time. One type of memory which has enjoyed widespread use in the data processing field is that of magnetic media disk memories.
In general, disk memories are characterized by the use of one or more magnetic media disks stacked on a spindle assembly and rotated at a high rate of speed. Each disk is divided into a plurality of concentric "tracks" with each track being an addressable area of the memory array. The individual tracks are accessed through magnetic "heads" which fly over the disk on a thin layer of air. Typically, the disks are two-sided with a head accessing each side. In operation, these magnetic recording heads recover digital information from the recorded media by detecting magnetic flux reversals written onto the media.
Because of the small spacings and narrow tolerances involved in rigid-disk recording systems, the most important properties needed in advanced media are generally of a mechanical nature. Substrate and coating surfaces must be smooth to reduce noise and to reduce head-to media spacing. Mechanical wear resistance and magnetic uniformity are highly important for all types of media, but especially so for thin films or thin particular coatings. This means that the texturing process, which provides uniform microscopic grooves across the surface of the disk, is crucial to magnetic recording systems with high information density.
Texturing improves the properties of the magnetic rigid-disk in several ways. First of all, texturing removes the possibility of a Johansson Block effect occurring between the recording head and the disk surface. The Johansson Block effect refers to the tendency of the magnetic recording head to stick to a perfectly flat substrate surface due to the relative vacuum formed in between. The grooves prevent a vacuum from forming by allowing air molecules to penetrate the head/disk interface. The grooves, therefore, are essential to avoiding cohesion between the disk and head which may prevent the drive spindle from turning after a standstill.
The microscopic grooves also act as a reservoir for loose organic particulate matter which may find its way onto the disk's surface. In this way, the grooves function as tiny ditches to drain away contaminants from the disk surface where they might interfere physically or electrically with the head-media interface.
Another purpose of texturing is to enhance the magnetic properties of the rigid-disk surface by reducing the radial component of magnetization while intensifying the circumferential component. A large circumferential component of magnetization results in better differentiation between adjacent tracks on the magnetic surface.
In the course of manufacturing a magnetic disk, the substrate is first plated with nickle to a thickness of about 0.5 to 1.0 thousands of an inch and polished to a mirror finish. Standard substrate materials for rigid-disk recording media include high-purity aluminium and aluminum (4-5%) magnesium alloys. These substrate materials provide a uniform smooth surface which permits close head-to-media spacing in addition to reducing substrate-induced noise.
The next manufacturing step involves the actual texturing of the disk surface. The purpose of texturing, as mentioned, is to improve the physical and magnetic properties of the recording surface. In the texturing process, numerous microscopic grooves are cut circumferential into the disk's surface using either a fixed-abrasive or free-abrasive medium. In general, the grooves measure approximately 12.times.12 microinches in dimension. Each groove is separated from its nearest neighbor by approximately 20-30 microns. (Practically, the grooves are not located on a true circumference of the disk. Rather, the grooves are cross-hatched--intersecting at an approximate 10 degree angle to each other.)
Most texturing equipment utilizes an abrasive mineral, such as silicon carbide or aluminium oxide, for cutting the grooves. The mineral is bonded to a mylar-backed tape which is then passed over a cylindrical load roller. The tape is mechanically forced against the surface of the disk by the load roller. Commonly, two load roller assemblies are positioned side by side to texture the front and back surfaces simultaneously. To facilitate the process, the rigid-disk substrate is often rotated against the tape/roller system at a high rate of speed.
Numerous variations to this basic process exist. For instance, often a liquid is supplied at the tape/disk interface to lubricate and/or cool the disk surface during the cutting process. Cross-hatching of the grooves may also be accomplished by mechanically oscillating the roller across the radius of the disk, e.g., from the inside diameter to the outside diameter. As is appreciated by practitioners in the art, the quality of the microscopic grooves is extremely dependent on a great many process variables which have remained relatively uncontrolled in prior approaches.
After the rigid-disk surface has been textured, a thin magnetic film is applied to the surface of the disk. The thin magnetic film comprises the actual recording media. Most magnetic films are nickel-cobalt alloys which are deposited by either electrical plating, chemical plating, evaporation or sputtering. The typical thickness of these films may range anywhere from 2 to 3 microinches.
Following the deposition of the magnetic media material, a protective overcoating (typically some sort of carbon compound) is sputtered onto the surface of the substrate. The overcoating is applied after the magnetic layer to provide abrasion resistance from the recording head. Buffing of the protective overcoat completes the processing of the magnetic rigid-disk.
There are a number of drawbacks associated with prior art texturing machines. For instance, prior approaches generally ignore the need for inline process control because of the difficulty of measuring and controlling each of the various process parameters involved. Parameters, such as tape speed, tape tension, applied force, etc., usually must be manually preset in prior art systems before the start of a processing cycle. In other words, the control of such systems is completely "open-loop" in nature.
Most often, tape speed and tension are established by a DC motor and drag clutch arrangement that is calibrated by hand. However, once the work on the disks has commenced, there is little way of knowing whether any of the relevant parameters have changed during the processing cycle. For this reason, previous finishing systems have been incapable of furnishing quality control information to the user on a real-time basis. Furthermore, due to the lack of automation and instrumentation, prior art systems have been unable to provide the user with a measure of the actual work being performed at the disk/tape interface. Therefore, what is needed is an automated finishing system which provides in-line process control features.
As will be seen, the present invention provides an automated rigid disk finishing system with "fly-by-wire" control. That is, each of the relevant processing parameters are remotely programmed by the user and thereafter controlled by the finishing system without the need for manual adjustment. The described system is thus capable of establishing tape speed and tape tension simultaneously, while providing a measure of the actual work being accomplished on the rigid disk surface. Employing these features, the invented system achieves real-time, in-line process control for the first time in a disk finishing system.